feat(firestore): add BSONRegex support

[ohmayr]

Adds the BSONRegex data type to support BSON regular expression field serialization in Google Cloud Firestore according to the official Firestore BSON specification.

• BSONRegex Container Class: Introduces google.cloud.firestore_v1.bson.BSONRegex to encapsulate regex patterns (pattern: str) and option flags (options: str | re.RegexFlag | int).
• Flag Normalization: Automatically normalizes Python re flag integers (re.IGNORECASE | re.MULTILINE) into sorted, unique BSON option flag strings (e.g., "im").
• Wire Serialization: Implements _to_map_value() converting BSONRegex into the reserved {"__regex__": {"pattern": pattern, "options": options}} wire map representation.
• SDK Surface Export: Exported on top-level package namespaces (google.cloud.firestore_v1 and google.cloud.firestore).

Fixes: b/562163604

[gemini-code-assist]

Code Review

This pull request introduces the BSONRegex class to represent BSON Regular Expression containers for Firestore, along with corresponding unit and system tests. The new class is integrated into the package's public API and supports pattern validation, option flag normalization (including conversion from Python re flags), serialization, and comparison operations. There are no review comments, and I have no feedback to provide.